Is the main difference that?
Interrupt coalescing (ethtool -C eth1 rx-usecs 0) - coalesce the received packets from different connections, i.e. increase bandwitdh, but increase the latency of the receive
Nagle algorithm (socket options = TCP_NODELAY) - coalesce the sent packets from the same connection, i.e. increase bandwitdh, but increasethe the latency of the send
Interrupt coalescing concerns the network driver: the idea is to avoid invoking the interrupt handler anew every time a network packet shows up. Instead, after receiving a packet, the NIC waits until M packets are received or until N microseconds have passed before generating an interrupt. Then the driver can process many packets at once. (Otherwise, with modern gigabit and 10-gigabit adapters, the processor would need to field hundreds of thousands or millions of interrupts per second, which can prevent the system from being able to accomplish much else.) As your link points out, there is (or at least may be) a cost of additional latency since the OS doesn't start processing a received packet at the earliest possible instant.
Nagle's algorithm is focused on reducing the number of packets sent by coalescing payload data from multiple packets into one. The classic example is a telnet session. Without Nagle, every time you press a key, the system has to create an entire new packet (min 64 bytes on Ethernet) to send one byte.
So the intent of interrupt coalescing is to support greater bandwidth utilization, while the intent of Nagle's algorithm is actually to produce lower bandwidth (by sending fewer packets).